Inappropriate Urination

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Hi...
We have two Bengalkittens, half-siblings (same father, different mothers), both male. One kitten is about six months old, the other is closer to eight months. They came home together from the breeder, and were ten and twelve weeks of age at the time. They were both neutered at eight-weeks of age.
I considered to the breeder to be an extremely responsible breeder. Her home was extremely clean and all cats were healthy and very well cared for. All kittens were seen by a vet several times from birth until old enough to go to their new homes. Of course, aside from being neutered, they were also up-to-date on their vaccines. They were sold with all veterinary records, registration papers, a contract, and a health guarantee. I researched the breeder and was pleansantly surprised to find that many of the cats have done quite well in the show world (even though I was only purchasing a "pet," it was nice to know that the breeder was breeding for quality, conformation, as well as temperment). And, I would not consider this breeder to be operating a "cattery."
The kittens were litter trained (using pine litter) at the breeders home. They did not seem to exhibit any behavioral issues during the transition period after coming into our home. The kittens were playful, very active, curious, etc.. As a side note, there are no other pets in the home.
In the beginning, we had a few instances of "accidents," but nothing that would have clued me in that there would be a future problem with litter box use. I was VERY wrong.
Over the course of the past few months, the kittens have become a nightmare when it comes to peeing in the house. There are four (4) litter boxes, two upstairs and two downstairs. The litter boxes are scooped daily and thoroughly cleaned once a week. They faithfully use the litter box to poo, and also use the litter box frequently to urinate; however, they also urinate LOTS of other places as well. It seems to be a problem that is increasingly getting worse. One of the cats seems to be a worse offender than the other, but both have been caught inappropriately urinating in the home.
They have urinated on our bed twice. They urinate on dirty laundry if it is on the floor. (I used to sort the laundry into piles on the laundry area floor, but now have to literally guard the dirty laundry and keep it locked up.) They will also urinate on clean laundry, especially if it is in the basket. So, I have to IMMEDIATELY put ALL of the laundry away the moment it comes out of the dryer, because if I set the basket down unattended for any length of time it will be peed on. They like to play with socks, which I have seen them drag into the living room or kitchen an pee on, so I have to be dilligent that my little boy (who is 5-years-old) doesn't leave his socks on the floor. They have peed on plastic bags, so bringing groceries requires locking the cats up until all of the groceries are put away. And, I recently discovered that they peed on all of my shoes that were in the bottom of my closet. They urinated on one of the chairs in our living room.
Although it sounds horrible, the problem seems to be somewhat intermittent. They might go weeks without peeing anywhere in the house, and then take a spell where they do it almost every day. Very unpredictable.
All areas that have been peed on have been cleaned very, very thoroughly. (The chair was thrown away.) And, additionally, they do not seem to have a preference for a particular spot nor do they tend to return to an old spot due to smell.
I currently free feed dry food and have clean, fresh water available at all times.
I have tried the Feliaway plugs (continue to use them) but see no benefit.
My next steps are to wean them onto canned cat food only over the next few weeks and consider transitioning to a different kind of litter. Or, making several different kinds of litter available at the same time to see if they have a preference. (I currently use pine clumping litter.)
There has been no change in the locations of their litter boxes or brand of litter. The boxes are standard open boxes, not covered.
Both cats have been checked by a vet several times. Clean bill of health, no UTIs, so there is no obvious medical cause for the behavior.
It is becoming a SERIOUS issue... and, despite my thorough cleanings, I think my house is starting to smell like a litter box. Not to mention the fact that we are having to throw away things that are rather expensive to replsce (i.e. chair, shoes, some articles of clothing that I was unable to remove the urine, etc.)
The cats are indoor-only. They are not declawed.

Please, PLEASE do not get your cats declawed. it is the MOST PAINFUL AND TRAUMATIC thing you could possible do to a cat.
I also have a Bengal who started spraying around the house when he was 18 months or so old. I think it was about the time my older cat died. I have not had any success so far with stopping him but have enclosed the backyard with cat mesh so he has freedom and safety but not allowed inside until bedtime when he and his companion sleep in the laundry. Wish I could be more help.